Known in the art is an apparatus to secure a tool in a holder (cf USSR Inventor's Certificate No. 589,085, issued on Sept. 2, 1975 and published on Jan. 25, 1978 in Bulletin "Discoveries, Inventions, Industrial Designs and Trademarks" No. 31), wherein the tool terminates in a shank made in the form of a body of revolution, fitted in a hole inside the holder. The apparatus comprises a pin placed in the holder hole and in a groove made on the tool shank. The groove is located in one plane on the curvilinear generatrix of the shank, and has side surfaces at an angle to its longitudinal axis. The pin effecting the attachment of the tool in the holder is arranged in the holder hole and in the groove on the tool shank in such a manner that the surface thereof contacts both side surfaces of the groove or fully fits the latter. The resultant radial force secures the tool in the holder, i.e. the former is reliably self-locked in the latter. However, the apparatus fails to ensure an adequate rigidity of the tool-holder system, because this rigidity is dependent on that of the shank, i.e. the male part. The diameter of the male part (shank) being always smaller than that of the female part (holder), the rigidity of the former is inferior to that of the latter, i.e. the rigidity of the tool-holder system in this case cannot be higher than that of the shank. Hence, said prior art apparatus may be used only for securing a tool which does not experience considerable loads in operation.